This invention relates to a detector for detecting intermediate products in raw materials or in wrapped finished products of food products or for detecting the presence of materials such as metals and other foreign matter in pharmaceutical products. This invention also relates to a detector for detecting in products a different quality from the inherent or standard quality of the actual product.
For manufacturers, such as food product manufacturers and pharmaceutical manufacturers, the presence of foreign matter in raw materials could cause damage to processing equipment. Also, the presence of foreign matter in finished products poses problems with regard to health and safety of the products. For this reason, detectors have long been used for inspections at the point at which raw materials for food products or pharmaceutical products are supplied to processing equipment and for inspection at the shipping point of wrapped finished products in order to determine whether or not foreign matter was present in the products.
As shown in FIGS. 19 and 20, this type of prior art detector is provided with a primary coil 3 which generates an alternating magnetic field when a high-frequency electrical current is supplied. Two secondary coils 4 and 5 are positioned inside this electromagnetic field. These coils 4 and 5 are wound in opposite directions from each other and are interconnected in series. In FIG. 19, the primary coil 3 is wound between and coaxially with the secondary coils 4 and 5. In FIG. 20, the primary coil 3 is wound in a plane parallel to the secondary coils 4 and 5. The detector utilizes the following phenomena which occur when the object being inspected is passed through the primary coil 3 and the two secondary coils 4 and 5 as indicated by arrow 31. If iron is contained in the object being inspected, the magnetic flux density will increase. The induced voltage of the secondary coil 4, which is closer to the point which the object is passing, will become higher than the induced voltage of the other secondary coil 5. Also, if a nonferrous metal is contained in the object, the eddy current occurring inside the nonferrous metal will cause a loss of the lines of magnetic force. As a result, the induced voltage of the secondary coil 4 will become lower than the induced voltage of the secondary coil 5.
Some food products may contain water or salt which may generate a relatively large signal even though they do not contain foreign matter. This signal represents the product characteristics of the material. If ferrous or nonferrous foreign matter is contained in these materials, the combination of the signal from this foreign matter and the product characteristics described above result in a composite signal. However, because the signal resulting from the foreign matter will be small if the foreign matter is small, there is little difference between the signal generated by the product characteristics of the product itself and the combination of this signal and that caused by the foreign matter. This makes it difficult to detect the foreign matter.
It is in consideration of problems such as this that detectors such as that described in Japanese Patent Provisional Publication Sho. 57-198880 have been proposed. In that detector, the oscillation signal from an oscillator is amplified via a phase adjuster to excite the primary coil. The difference in the induction voltages generated at the two secondary coils when the product passes through is used as the detection signal. This signal is first amplified and then split into two output signals. One of these output signals is detected using a chopper which is synchronized with the aforementioned oscillation signal to obtain an in-phase output signal. In addition, the other of these output signals is detected by a chopper which is provided with a phase difference of 90.degree. with respect to the aforementioned oscillation signal to obtain a quadrature output signal. In this method, each of the obtained output signals passes through its own filter and is independently compared by a level comparator. Particularly for a product having a material effect, this type of detector is adjusted by the phase adjuster to the phase angle at which the material effect is minimum (to which the signal of the material effect is 90 degrees). The phase adjustment is done in a trial and error method, which requires skill of the operator. It is therefore difficult for the skilled operator to set the detector to an accurate phase. As a result, the detector may often be used with low accuracy. In addition, the detector cannot detect a foreign matter producing a signal in the reverse phase to the signal of a material effect (in the decreasing direction).